In a wireless communication system, it is critical to design a mechanism for allowing a remote subscriber station (SS) to access the network by sending an access signal to a Base Station (BS). The access signal fulfills important functions such as requesting resource allocation from the BS, alerting the BS of the existence of the SS that is trying to enter the network, and initiating a process that allows the BS to measure some parameters of the SS (e.g., timing offset caused by propagation, transmit power, etc.) that must be maintained and adjusted constantly in order to ensure a non-interfering sharing of the uplink resource. Unlike ordinary data traffic that is sent using scheduled resources that are allocated by the BS to the SS, such an access signal is often transmitted in an unsolicited manner. Therefore, this process is often referred to as a random access. Sometimes the process is also referred to as “ranging”, as used in the Institute of Electrical and Electronic Engineers (IEEE) 802.16 standards, because the access signal can help the BS to measure the propagation distance from the SS (thus, its range). A parameter known as a timing advance offset is used by the SS to advance its transmission relative to the reference timing at the BS so that the signals from all the SS's appear synchronized at the BS (i.e., uplink timing synchronization). Once uplink timing synchronization is achieved, the SS orthogonality is ensured (i.e., each SS occupies its own allocated sub-carriers without interfering with other SS). In this specification, the terms “access”, “random access”, and “ranging” will be used interchangeably to describe these processes and also to describe the signal transmitted by the SS to initiate the access process.
The random access or ranging process includes an initial/handover ranging function for synchronizing an SS with a BS during the initial network entry or re-entry and during cell handoff, a periodic ranging function for maintaining SS synchronization, and a bandwidth request function that allows each SS to request uplink bandwidth allocation. These uplink ranging functions fulfill very important tasks that can significantly influence the user experience. For example, the bandwidth request ranging performance directly impacts the access latency perceived by a user, especially during communication sessions (e.g., HTTP) that consist of sporadic packet traffic that requires fast response, in which case high detection and low collision probabilities of the access request are very desirable. In another example, robust detection of an initial ranging signal is essential in order to allow a user to quickly enter the network or to be handed over to a new serving sector. Reliable extraction of the accurate timing offsets from the initial ranging signals is also critical for achieving uplink synchronization that ensures user orthogonality (i.e., to make sure that each SS occupies its own allocated sub-carriers without interfering with other SS). Other important information that the BS needs to extract from ranging includes power measurement, frequency synchronization, and channel impulse response estimation, etc. Therefore, there is a need for an efficient and flexible air interface mechanism that enables fast and reliable user access to the network.